The giant impact that created the Moon also seeded Earth with life-giving carbon

Why did our planet turn out
like it did — a temperate, life-supporting place? Why weren’t the
volatile elements (sulfur, carbon, and the like) in our planet boiled
away into space or locked in the core, like on Mars or countless other
cold, rocky exoplanets? A team of scientists from Rice University have
published a new explanation for the chemistry of our planet’s surface,
and it has to do with the giant impact hypothesis.

The story goes like this: About four and a
half billion years ago, the Earth first settled together into a planet.
Only about a hundred million years afterward, it probably had an oblique
collision with a slow-moving, massive body somewhere between the sizes
of Mercury and Mars. According to the giant impact hypothesis, that
massive body was Theia, and the impact sent a bolus of mashed-together
molten planet sloshing into space, where it coalesced into the Moon.
Visualize water droplets combining in zero gravity, except with two
huge globs of partially melted rock glowing hot through the cracks.

This is all extremely difficult to verify,
because beyond about 4 billion years ago, the Earth’s face has been so
changed by its own tectonic activity that all we have left to bear
witness to the aptly named Hadean Eon are scattered zircon crystals
embedded in other, younger rocks. But zircons don’t give us a conclusive
history of our planet through deep time, other than having the
chemistry they do and being as old as they are. They don’t tell us about
what happened before the Late Heavy Bombardment. But these scientists
think there are still clues written in the bulk chemical composition of
the planet as a whole.

An
artistic conception of the early Earth, showing a surface pummeled by
large impacts, resulting in extrusion of deep seated magma onto the
surface. At the same time, distal portions of the surface could have
retained liquid water. Credit: NASA/Simone Marchi/SwRI.

Before the oxygen catastrophe about 2.3 billion years ago, our planet’s atmosphere
was a reducing environment — which is to say, the planet’s chemistry
was dominated by sulfur compounds instead of oxygen. But the
cyanobacteria changed everything. Their shiny new talent for
photosynthesis came with a catch: they excreted diatomic oxygen, good
old O2 gas, as a waste
product. Oxygen built up in the environment to such concentrations that
it permanently changed the atmospheric composition from a reducing,
sulfurous atmosphere to the breathable, oxidizing one we have now.

Atmospheric chemistry is really important
here, because it totally changes the path of chemical reactions on the
planet. In anoxic environments, for example, iron acts differently than
its familiar oxidation (rusting) behavior. Under anoxic conditions, like
the sulfurous environment on the still-molten early earth, metals like
iron tend to form sulfides and carbides, which are heavy enough to sink.
But, the scientists contend, sufficient concentrations of silica can
force metallic compounds to play hot-potato with their functional
groups, accepting silica and rejecting carbon to the mantle. Dumping a
ton of silica into the planet’s metal-sulfide and carbide core could
switch the equilibrium and induce the formation of less-dense metal
silicates and carbonates, like those found in terrestrial igneous and
sedimentary rocks. This is where the great impact comes in.

An embryonic planet — one that had been around
long enough to make carbon-rich surface rocks, long enough for the
silicon to sink to its core — could explain the mixing we see. If a
young, slow-moving, Mercury- or Mars-sized planet
collided with our own, the two might not completely mix; the lighter,
cooler mantles would be able to intermingle, well excluded from the
relatively liquid core and its different chemistry balance.

“Because it’s a massive body, the dynamics
could work in a way that the core of that planet would go directly to
the core of our planet, and the carbon-rich mantle would mix with
Earth’s mantle,” explains Rajdeep Dasgupta, coauthor of the study.

Given Mercury has a silicon-dominated core
chemistry, the idea of finding a nearby Mercury-sized planet with a
silicon-rich core isn’t too far-fetched. The sulfur chemistry of the
early Earth provides all the conditions required for this hypothesis.
And the Late Heavy Bombardment would have done the rest of the surface
mixing to achieve the surface chemistry we have, with the planetary
carbon and sulfur budget we have. As with so many things in science,
only further observation will help to form a conclusion. But it’s really
something to think about, the idea Earth only became this lush, lovely
planet after total surface-melting cataclysm.